alpha-Crystallin and small heat-shock proteins (sHSP) show sequence similarity along a stretch of 100 amino acids referred to as the alpha-crystallin domain. The presence of this domain is associated with the ability of sHSP to assemble into oligomeric structures and with their function as molecular chaperones. Site-directed spin labeling was used to investigate quaternary interactions along a conserved sequence in the alpha-crystallin domain of alphaA-crystallin and heat-shock protein 27 (HSP 27). In previous work, it was demonstrated that this sequence in alphaA-crystallin and HSP27 forms a beta-strand involved in subunit contacts. In this study, the symmetry and geometry of the resulting interface were investigated. For this purpose, the pattern of spin-spin interactions was analyzed and the number of interacting spins was determined in alphaA-crystallin and HSP27. The results reveal a two-fold symmetric interface consisting of two beta-strands interacting near t heir N-terminal in an antiparallel fashion. Remarkably, subunit interactions along this interface persist when the alpha-crystallin domians are expressed in isolation. Because this domain in alphaA-crystallin forms dimers and tetramers, it is inferred that interactions along this interface mediate the formation of a basic dimeric unit. Taken together, our results demonstrate that subunit interactions in the alpha-crystallin domain of mammalian small heat-shock proteins assemble a basic building block of the oligomeric structure. Sequence divergence in this domain results in variations in the size and symmetry of the quaternary structure between distant members of the small heat-shock protein family.